Preference for plants damaged by conspecific larvae in ovipositing cabbage root flies: influence of stimuli from leaf surface and roots

In laboratory dual-choice assays females of the cabbage root fly, Delia radicum, prefer for oviposition plants with roots damaged by conspecific larvae to undamaged controls. Cauliflower and kale plants were inoculated with root fly eggs (25 per plant) and the hatching larvae were allowed to feed on the roots for various periods of time (1–17 days). After 4 (cauliflower) or 5 (kale) days of larval feeding the oviposition preference was most pronounced and flies laid between 64% and 68% of their eggs near plants with damaged roots. Later, with increasing damage but fewer surviving, and thus actively feeding, larvae, the magnitude of the preference declined. The preference for plants already damaged by conspecific larvae may contribute to the previously observed aggregated distribution of D. radicum eggs in Brassica crop fields.Further experiments revealed that the sensory cues inducing this oviposition preference originate from the complex consisting of the damaged roots, the surrounding substrate (soil) and associated microbes, rather than from the aerial plant parts. In choice assays using the root-substrate complex of damaged and control plants (aerial parts removed), the observed preference for damaged roots was similar to that found for the entire plant but was more pronounced. The damaged roots alone, compared to control roots, received up to 72% (cauliflower) and 75% (kale) of the eggs. By contrast, surrogate leaves sprayed with methanolic leaf surface extracts from the most preferred plants which had been damaged were not discriminated from surrogate leaved sprayed with extracts of the respective control plants. Analysis of glucosinolate levels in methanolic leaf surface extracts revealed that root damage resulted in enhanced concentrations of indole-glucosinolates on the leaf surface in kale but not in cauliflower. Although indole-glucosinolates are oviposition stimulants for the cabbage root fly, the induced changes were apparently too small to influence oviposition behaviour.     

Effects of plant chemical extracts and physical characteristics of Apium graveolens and Chenopodium murale on host choice by Spodoptera exigua larvae

Choice tests with whole plants and leaf discs indicated that fourth instar Spodoptera exigua (Hübner) (Noctuidae: Amphypyrini) were found more frequently and ate significantly more of the weed Chenopodium murale than the associated crop plant Apium graveolens. In order to explain the preference, plant extracts, plant volatiles, soluble protein concentrations, water contents, and leaf toughness of the two plants were investigated. Bioassays of aqueous methanol (90%) and hexane extracts of leaves on cellulose discs indicated that neither attractants in C. murale nor repellents in A. graveolens could account for the observed preference. No significant difference could be found between the effects of plant volatiles from C. murale, A. graveolens and a control on larval dispersal by S. exigua. Selective feeding for higher levels of proteins also was not a factor, because A. graveolens had nearly twice the soluble protein of C. murale. Water content was approximately 6% higher (by weight) in C. murale than A. graveolens but most polyphagous larvae do not typically show compensatory feeding for water alone. However, the potentially related characteristic of leaf toughness was significantly different, with A. graveolens exhibiting 1.53 times the toughness of C. murale. Studies comparing five types of larval behavior on both plant species showed that the time spent in swallowing behavior was significantly greater on the tougher A. graveolens leaves relative to C. murale. To test the hypothesis that leaf toughness was affecting larval host choice, both plants were finely ground and incorporated into agar blocks. No differences in feeding behavior were detected. The implications of leaf toughness for larval diet and host choice are discussed.     

POLLEN-TUBE COMPETITION AND MALE FITNESS IN HIBISCUS MOSCHEUTOS

The stigmas of animal-pollinated flowers often capture more pollen than is needed to fertilize all available ovules, and mixed-donor pollen loads are probably common. When this is the case, variation in average pollen-tube growth rates can potentially affect the number of seeds sired by a given plant. Despite considerable interest in effects of postpollination processes on male fitness, little is known about the extent of variation in pollen performance among plants from natural populations. To examine this question in Hibiscus moscheutos (rose mallow), we conducted mixed-donor hand-pollination experiments with 39 pollen donors bearing distinctive isozyme markers. Pairs of competing donors were compared on sets of 11 to 15 recipient plants per pair. These donors often differed in the proportions of seeds they sired, with the maximum deviation from an expected ratio of 50:50 being 68:32. Furthermore, three intensively studied plants exhibited consistent trends in relative pollen performance when each was tested against (1) the same three competitors, and (2) groups of 14 competitors chosen at random from the study population. In a separate experiment, we investigated the effects of salinity stress and high soil nutrients on pollen performance. These environmental factors had anticipated effects on leaf production, flower production, and petal length, but style length and (most importantly) the number of seeds sired relative to a standard pollen donor were not affected. In summary, this study provides the strongest evidence to date that pollen-tube competitive ability varies among coexisting plants and may be an important component of male fitness in plants.     

FEMALE CONTROL OF SPERM TRANSFER AND INTRASPECIFIC VARIATION IN SPERM PRECEDENCE: ANTECEDENTS TO THE EVOLUTION OF A COURTSHIP FOOD GIFT

Manipulation of ejaculates is believed to be an important avenue of female choice throughout the animal kingdom, but evidence of its importance to sexual selection remains scarce. In crickets, such manipulation is manifest in the premature removal of the externally attached spermatophore, which may afford females an important means of postcopulatory mate choice. We tested the hypothesis that premature spermatophore removal contributes significantly to intraspecific variation in sperm precedence by (1) experimentally manipulating spermatophore attachment durations of competing male Gryllodes sigillatus and (2) employing protein electrophoresis to determine the paternity of doubly mated females. The relative spermatophore attachment durations of competing males had a significant influence on male paternity, but the pattern of sperm precedence deviated significantly from the predictions of an ideal lottery. Instead, paternity data and morphological evidence accorded best with a model of partial sperm displacement derived here. Our model is similar to a displacement model of Parker et al. in that sperm of the second male mixes instantaneously with that of the first throughout the displacement process, but the novel feature of our model is that the number of sperm displaced is only a fraction of the number of sperm transferred by the second male. Regardless of the underlying mechanism, female G. sigillatus can clearly alter the paternity of their offspring through their spermatophore-removal behavior, and employ such cryptic choice in favoring larger males and those providing larger courtship food gifts. We discuss how female control of sperm transfer and intraspecific variation in sperm precedence may be important precursors to the evolution of gift giving in insects.     

Identification of genetic factors for alachlor tolerance in maize by molecular markers analysis

Genetic factors controlling tolerance to the herbicide Alachlor in maize were localised by means of two different strategies. In the first approach, backcross (BC) plants, derived from pollen which had been subjected to selective pressure for resistance to the herbicide, were analysed for segregation distortion at 47 RFLP loci and compared to BC plants obtained from non-selected pollen. Preferential transmission of five chromosomal regions where putative QTLs (Quantitative Trait Loci) are localised was revealed in the BC plants from selected pollen. A second approach was based on a classical linkage analysis for segregation of the same set of RFLPs and factors controlling the trait, in a BC population of 210 individuals, by means of regression analysis. This study detected seven significant loci in four genomic regions. Overall, two loci revealed both segregation distortion and association with the expression of the trait, indicating linkage to genes expressed in both gametophytic and sporophytic phase. Three chromosomal regions appeared to carry factors involved in plant tolerance to Alachlor which are not expressed in pollen. Conversely, three loci were linked to factors selectable in pollen, but did not reveal significant association with tolerance in the plant in the segregating populations.     

Functional morphology of genitalia of four species of julidan millipedes (Diplopoda: Nemasomatidae; Julidae)

The functional anatomy of the genitalia of Nemasoma varicorne (Nemasomatidae), Brachyiulus lusitanus, Unciger foehdus and Cylindroiulus boleti (Julidae) was investigated by shock freezing of animals in copula followed by serial semithin sectioning of the engaged genitalia. The species show conspicuous differences in the functional mechanism of their genitalia. In M varicorne and B. lusitanus the vulvae remain within the vulval sac during copulation while in U.foetidus and C. boleti parts of the gonopods (promerite and mesomerite) form clasper-like structures that pull out the vulvae from the vulval sacs. With the exception of C. boleti all investigated species have a 'central funnel' on the vulva which leads into the receptaculum seminis. The sperm-transferring part of the male gonopods (solenomerite) is introduced into this funnel during copulation. In B. lusitanus and C. boleti a projection of the posterior gonopods (end-projection, brachite) fits into a slit anterior to the openings of the receptacula. The results are discussed with regard to sexual selection theory and a hypothesis is proposed that explains the evolutionary change of millipede genitalia by a combination of female choice and sperm competition phenomena.     

Explanatory pluralism in evolutionary biology

The ontological dependence of one domain on another is compatible with the explanatory autonomy of the less basic domain. That autonomy results from the fact that the relationship between two domains can be very complex. In this paper I distinguish two different types of complexity, two ways the relationship between domains can fail to be transparent, both of which are relevant to evolutionary biology. Sometimes high level explanations preserve a certain type of causal or counterfactual information which would be lost at the lower level; I argue that this is central to the proper understanding of the adaptationist program. Sometimes high level kinds are multiply realised by lower level kinds: I argue that this is central to the understanding of macroevolution.     

The extended replicator

This paper evaluates and criticises the developmental systems conception of evolution and develops instead an extension of the gene's eye conception of evolution. We argue (i) Dawkin's attempt to segregate developmental and evolutionary issues about genes is unsatisfactory. On plausible views of development it is arbitrary to single out genes as the units of selection. (ii) The genotype does not carry information about the phenotype in any way that distinguishes the role of the genes in development from that other factors. (iii) There is no simple and general causal criterion which distinguishes the role of genes in development and evolution. (iv) There is, however, an important sense in which genes but not every other developmental factor represent the phenotype. (v) The idea that genes represent features of the phenotype forces us to recognise that genes are not the only, or almost the only, replicators. Many mechanisms of replication are involved in both development and evolution. (vi) A conception of evolutionary history which recognises both genetic and non-genetic replicators, lineages of replicators and interactors has advantages over both the radical rejection of the replicator/interactor distinction and the conservative restriction of replication to genetic replication.     

Evolutionary plasticity in prokaryotes: A panglossian view

Enzyme directed genetic mechanisms causing random DNA sequence alterations are ubiquitous in both eukaryotes and prokaryotes. A number of molecular geneticist have invoked adaptation through natural selection to account for this fact, however, alternative explanations have also flourished. The population geneticist G.C. Williams has dismissed the possibility of selection for mutator activity on a priori grounds. In this paper, I attempt a refutation of Williams' argument. In addition, I discuss some conceptual problems related to recent claims made by microbiologists on the adaptiveness of molecular variety generators in the evolution of prokaryotes. A distinction is proposed between selection for mutations caused by a mutator activity and selection for the mutator activity proper. The latter requires a concept of fitness different from the one commonly used in microbiology.     

Introgression of the avian naked neck gene assisted by DNA fingerprints

Theoretical predictions suggest that DNA markers can be useful tools for genomic selection in gene introgression programmes. An experiment was carried out to evaluate the efficiency of using multi-locus DNA markers in an introgression programme designed to transfer the naked neck gene from a donor to a recipient chicken line. The donor line was a commercial egg layer chicken stock heterozygous at the naked neck locus (Na/na⁺), while the recipients were from a Cornish broiler line. These two lines differ markedly in their average body weight, a quantitative trait that can also represent the comprehensive differences between the genomes of the two lines involved. Three groups of naked neck BC₁ individuals were selected according to the following criteria: (i) low band-sharing with their donor grandsires evaluated by multi-locus DNA markers, (ii) high body weight at six weeks of age, and (iii) selection at random as a control group. Birds from each of these groups were mated at random to individuals from the heavier Cornish line to produce three groups of BC₂ individuals whose body weights were recorded weekly from three to seven weeks of age. Results indicated that BC₂ birds obtained from BC₁ parents selected for band-sharing levels and those selected for body weight, performed equally well at 4–7 weeks of age; both were 3.1–3.9% heavier than birds from the randomly selected group. The additional genome recovery of the heavier broiler line, obtained by DNA markers, was found to be in agreement with theoretically predicted values.